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Big Advance for Small PABX
DF Griffiths Post Office Telecommunications Journal Spring 1978
AS THE WORLD of commerce grows increasingly dependent on telecommunications in day-to-day business, it is inevitably ever more demanding in the range of features it expects to see offered. Nowhere is this more evident than in the field of the Private Automatic Branch Exchange (PABX), usually the focal point for telecommunications within any organisation.
Whatever the size - whether it be a small firm of solicitors with a single PABX simply connected to the public network, or a large multi ·.national corporation with a complex private network - the demands will be similar. It is only the scale which differs.
In recent years the business customer with a requirement of more than about 100 extensions has enjoyed the benefits of many improvements in the larger proprietary PABX field but until recently developments in smaller PABX equipment - for which the Post Office is solely responsible - have been less marked.
In an effort to remedy this the Post Office began a feasibility study which led to a design it later adopted as the basis for its new rental range PABX. Known as Customer Digital Switching System No. One (CDSS1), it fulfills the need for a modern, compact re placement for the existing Strowger based small PABXS. It uses Post Office technology which is among the most advanced of its kind in Europe and a target date was set for 1980.
But consider first the background in more detail. Customer requirements include a comprehensive range of extension facilities such as enquiry, transfer, call diversion and abbreviated dialling. Usually the customer has minimal accommodation and requires the equipment to blend-in with normal office type environments. He also wants an attractive design for the operator's console - and asks for considerable flexibility coupled with ease of operation. Many of these demands can only be met-economically by employing some of the most advanced switching system technology available. The existing Strowger-based small PABXS and their corresponding extension capacities are the PABXS 5 and 6, serving up to 20 extensions, the PABX 1, serving up to 50 extensions, and the PABX 7, serving up to 100 extensions. The Post Office markets them on a rental basis and has full operational responsibility for all the 40,000 or so such systems which are now in service. Features offered by the current rental systems are basic, and often a dedicated equipment room is needed to accommodate the switching equipment and ancillaries. Installation methods for the existing rental PABXS tend to be time consuming and demanding on labour, and the Post Office is often faced with the costly exercise of changing systems as the customer's requirements grow. So it was obvious that something had to be done. Ater a study of available PABX systems and capabilities it was decided to conduct an in-house study on the problems and advantages of using Time Division Multiplex (TDM) switching with Stored Programme Control (spc) for a small PABX The design of CDSS1 which resulted was the product of a joint team of engineers in the Development and Research Departments of Telecom munications Headquarters (THQ). A production engineering contract was placed with Plessey Telecommunications Ltd and the General Electric Company Telecommunications Ltd, and the first pre-production trial sys tems are planned for early 1979. CDSS1 is designed to serve up to 120 extensions with MF keyphones or ordinary dial phones. It will also carry 24 exchange lines and 16 auxiliaries such as tone receivers and inter-PBx circuits.
The operator's console is discreet and compact, designed to match the projected new range of Post Office instruments, and offers speedy call handling in any kind of environment. Both the operator's console and the central equipment unit (CEU) have been designed to be compatible with modern offices, and there is a dramatic reduction in the amount of space occupied compared with, say, the PABX 7 which has fewer extensions.
The operator's console is discreet and compact, designed to match the projected new range of Post Office instruments, and offers speedy call handling in any kind of environment. Both the operator's console and the central equipment unit (CEU) have been designed to be compatible with modern offices, and there is a dramatic reduction in the amount of space occupied compared with, say, the PABX 7 which has fewer extensions.
Central to the success of CDSS1 is the use made of one of Europe's most advanced microelectronic chips, known as a single channel codec (coder/decoder), developed within the Post Office Research Department at Martlesham. Because of their cost, present methods of converting analogue speech signals to digital form are commonly shared between 24 and 30 channels. The predicted lower cost of the new Large Scale Integrated (LSI) codec means that it can be used on only one channel and may therefore be used on individual lines connecting telephone customers to their local exchanges.
Its use on CDSS1 means that a single printed wiring board measuring about 200 mm (8 in) high and 320 mm (12.7 in) deep accommodates all the circuitry needed for four extension lines, including the four codec chips. Such a high extension packing density means that the line cards, switching and control equipment, power supply and common services for a full size system can all be housed in a single free standing cabinet measuring about 1.6 m (5 ft) high by 0.6 m (2 ft) square - only slightly taller than a standard four-drawer filing cabinet.
The control shelf is at the top, the power shelf at the bottom and identical line unit shelves in between. The control and line unit shelves house printed wiring boards which are connected via high density connectors to wire-wrapped shelf backplanes at the rear. Along with the Control Processing Unit (cpu) and memory the top shelf also. houses the digital switch, signal ling access; system clock, conference card and tone generation. The time switch itself is non-blocking and is realised on two printed wiring boards. An equivalent space switch using standard Post Office reed relays would measure over 6.5 m (21 ft) square.
Apart from the switch there are two further areas where use of fully digital techniques has resulted in dramatic space saving. One of these is in tone generation where a single printed wiring board provides up to 32 different tones pre-cadenced as necessary and the other is in the conference unit. Here a single printed wiring board can perform simultaneous conference calls for up to 32 parties. Each of the five identical line unit shelves is designed to accommodate a maximum of 32 ports comprising six 4 port cards and four 2 port cards. Any inlet/outlet combination in CDSS1 having access to the switch is called a port. In configuring a particular customer installation the allocation of ports is fully flexible, which means that even the most unusual combinations of extensions, exchange lines and so on can be catered for. A shelf would typically cater for 24 extensions, SIX exchange/inter-PBX lines and two auxiliaries (MF4 receivers etc). Growth of the system is thus modular and in a downwards direction from the control shelf. The addition of a new line unit shelf merely requires installation of the line card frame and associated cards, connection of the shelf wiring harness to the shelf above, connection to the cabinet power bus and plugging-in the appropriate distribution cables.
A crucial module in CDSS 1 is the shelf multiplex card which appears at one end of each line unit shelf. It is here that the serial data streams from each of the 32 ports on a line shelf are multiplexed. They are converted into two serial data streams, one for speech and one for signalling. These data streams are then passed via the shelf wiring harness to the digital switch and signalling access cards on the control shelf. The system thus has the inherent and powerful capability of controlling signalling and speech connections entirely independently.
A crucial module in CDSS 1 is the shelf multiplex card which appears at one end of each line unit shelf. It is here that the serial data streams from each of the 32 ports on a line shelf are multiplexed. They are converted into two serial data streams, one for speech and one for signalling. These data streams are then passed via the shelf wiring harness to the digital switch and signalling access cards on the control shelf. The system thus has the inherent and powerful capability of controlling signalling and speech connections entirely independently.
CDSS 1 is powered direct from the mains and makes use of modern switching-mode principles to reduce size and optimise efficiency. Short term standby power will be available as a customer option. As far as the customer is concerned the front end is the operators console and great care has therefore been taken to design a unit which is not only pleasing in appearance but also versatile in performance. Considerable improvements in reliability are achieved by replacing the normal mechanical keys by capacitive touch keys.
The replacement of key tops by an overall plastic finger plate what PO Telecommunications Journal. Spring 1978 reduces the usual spares problem and blends in with the attractive low profile design. To optimise on flexibility without overloading the main CDSS1 processor, the operator's console incorporates its own microprocessor and associated programme memory. This flexibility means the console can be used to check out a faulty system to find inmost cases down to card level - failure has occurred. Thus when reporting a fault, advice can be given to the service engineer so that he can bring along the correct replacement card. A further use for the console is in entering customer system data and extension service changes. This avoids the expense of a dedicated terminal for the purpose and allows the customers to make operational changes without contacting the Post Office. On-site maintenance of CDSS1 is normally restricted to finding and replacing a faulty card. A number of continuously run self-checking routines are built into the system to ensure, for example, continuity of speech and signalling paths. A faultsman's telephone may also be used. In the long term it would be possible to provide for fault diagnosis from a remote maintenance centre. The provisioning procedure for CDSS1l can be managed by means of computer based records. Using this technique a customer's ordering requirement in terms of system size and facilities is entered into the computer where it is used to derive the necessary operational data. This includes such information as stock lists, installation charges and rental, initial customer data base and user instructions. The whole process can thus be automated to such an extent that the time between placing an order and delivery to site would always be kept to the minimum possible. As with most new stored programme controlled PABXS, CDSS1 offers the customer a large range of facilities most of which need only software to implement. Two typical facilities available to extensions are "call-back" - which enables an extension calling an engaged extension to be rung and connected when the latter becomes free, and "call diversion" - which enables an extension to divert incoming calls to another extension. The first customer trial was due to take place in late Spring and will be followed by pre-production trials The single channel codec, one of Europe's most advanced microelectronic chips and central to the success of CDSS1 . around the end of the year. Much has been achieved but even more remains to be tackled if by the scheduled date of 1980 business customers are to see their demands being met in the small PABX field.
The replacement of key tops by an overall plastic finger plate what PO Telecommunications Journal. Spring 1978 reduces the usual spares problem and blends in with the attractive low profile design. To optimise on flexibility without overloading the main CDSS1 processor, the operator's console incorporates its own microprocessor and associated programme memory. This flexibility means the console can be used to check out a faulty system to find inmost cases down to card level - failure has occurred. Thus when reporting a fault, advice can be given to the service engineer so that he can bring along the correct replacement card. A further use for the console is in entering customer system data and extension service changes. This avoids the expense of a dedicated terminal for the purpose and allows the customers to make operational changes without contacting the Post Office. On-site maintenance of CDSS1 is normally restricted to finding and replacing a faulty card. A number of continuously run self-checking routines are built into the system to ensure, for example, continuity of speech and signalling paths. A faultsman's telephone may also be used. In the long term it would be possible to provide for fault diagnosis from a remote maintenance centre. The provisioning procedure for CDSS1l can be managed by means of computer based records. Using this technique a customer's ordering requirement in terms of system size and facilities is entered into the computer where it is used to derive the necessary operational data. This includes such information as stock lists, installation charges and rental, initial customer data base and user instructions. The whole process can thus be automated to such an extent that the time between placing an order and delivery to site would always be kept to the minimum possible. As with most new stored programme controlled PABXS, CDSS1 offers the customer a large range of facilities most of which need only software to implement. Two typical facilities available to extensions are "call-back" - which enables an extension calling an engaged extension to be rung and connected when the latter becomes free, and "call diversion" - which enables an extension to divert incoming calls to another extension. The first customer trial was due to take place in late Spring and will be followed by pre-production trials The single channel codec, one of Europe's most advanced microelectronic chips and central to the success of CDSS1 . around the end of the year. Much has been achieved but even more remains to be tackled if by the scheduled date of 1980 business customers are to see their demands being met in the small PABX field.
Mr D. F. GriHiths is Head of Group in Telecommunications Development Department and project leader for production engineering and CDSS1.
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